Efimova was suspended for doping in 2014 and later tested positive for meldonium but was not suspended because she said she stopped taking it before it became a banned substance.

After Efimova took first in her prelim in Rio, she held up her finger to say she's No. 1. King, who was awaiting her own heat, was caught on camera wagging her finger at Efimova on TV.

NBC

Later, after King won her trial, she once again wagged her finger, then said after the race, "You know, you're shaking your finger No. 1 and you've been caught for drug cheating, I'm just not, you know, not a fan."

In the 11 months since the Olympics, however, it seems the tension between the two hasn't died down and has carried over to the World Championships in Budapest, Hungary.

Efimova took first in a preliminary heat for the 100m breaststroke, missing the world record by .01 seconds on Monday. Afterward, she turned to the crowd and wagged her finger, an obvious nod to King. King later told reporters, "I'm always looking at the results from the heat before. I saw a little finger wag. I saw it. It's just motivating me more, so that's okay."

On Wednesday, King was ready to face off against Efimova. Before the race, King stared down Efimova while Efimova warmed up, looking straight ahead.

King then got her revenge, beating Efimova to take gold in the final, setting a new world record in the process. Afterward, King shot a brief glance at Efimova.

King said after the race, "I couldn't have asked for a better race. I've been waiting on that time for a while."

American swimmer Katie Meili took silver while Efimova took bronze.

According to Nick Zaccardi of Olympic Talk, there may be more fireworks to come — the two will go head-to-head in the 200m breaststroke and 50m breaststroke finals. They're ranked Nos. 1 and 2 in the events, according to Zaccardi. We'll be on watch for more finger-wags and stare-downs.